Techniques such as polyclonal B cell activation with pokeweed mitogen (PWM) and polymerase chain reaction (PCR) analysis have documented the existence of simian immunodeficiency virus (SIV)-and human immunodeficiency virus type 1-seronegative but infected humans and nonhuman primates. To establish whether blood from such seronegative but PWM- and PCR-positive monkeys can transmit infection, naive macaques were transfused with whole blood (n = 2) or cultured cells and supernatant fluid (n = 2) from two seronegative but PWM- and PCR-positive sooty mangabeys. After transfusion, three of the four recipients seroconverted, and peripheral blood mononuclear cells from all four recipients secreted SIV-reactive antibodies upon polyclonal activation in vitro and were SIV-positive by PCR that used highly specific gag primer pairs and probe. In addition, CD8+ cells from all four recipients markedly inhibited replication of SIV in autologous cells in vitro. These data suggest caution in the sole use of serologic tests for the detection of retroviral infection and document the ability of such blood samples to transmit infection.